Active Acoustic Intensity Research Articles

Active intensity vortex and stagnation point singularities in a shallow underwater waveguide.

Vector acoustic properties of a narrowband acoustic field are observed as a function of range from a source towed in waters of depth 77 m on the New England Mud Patch. At the source frequency (43 Hz), the waveguide supported three trapped modes, with mode 2 weakly excited owing to the towed source depth. The receiving sensor was positioned 1.45 m above the seafloor with a sampling range aperture of 2500 m. The vector acoustics observations enabled study of vortex regions that encompass two singular points for active acoustic intensity: the vortex point, which is co-located with a dislocation, and stagnation point. Interpretative modeling, based on the normal modes and using a geoacoustic model consistent with those emerging from studies conducted at this location, is in agreement with these measurements. Model-data comparisons were based on the first-order variables of acoustic pressure and velocity along with inverse Hankel transforms, which yield normalized horizontal wavenumber spectra, and second-order variables in the form of horizontal and vertical intensity as well as non-dimensional intensity-based ratios. These measures provide a degree of observational confirmation of some vortex region properties. Both observations and modeling point to a gradual deepening of such regions with increasing range owing to sediment attenuation.

A coherence-based phase and amplitude gradient estimator method for calculating active acoustic intensity

The phase and amplitude gradient estimator (PAGE) method [Thomas, Christensen, and Gee, J. Acoust. Soc. Am. 137, 3366-3376 (2015)] has been developed as an alternative to the traditional p-p method for calculating energy-based acoustic measures such as active acoustic intensity. While this method shows many marked improvements over the traditional method, such as a wider valid frequency bandwidth for broadband sources, contaminating noise can lead to inaccurate results. Contaminating noise degrades performance for both the traditional and PAGE methods and causes probe microphone pairs to exhibit low coherence. When coherence is low, better estimates of the pressure magnitude and gradient can be obtained by using a coherence-based approach, which yields a more accurate intensity estimate. This coherence-based approach to the PAGE method, known as the CPAGE method, employs two main coherence-based adjustments. The pressure magnitude adjustment mitigates the negative impact of uncorrelated contaminating noise and improves intensity magnitude calculation. The phase gradient adjustment uses coherence as a weighting to calculate the phase gradient for the probe and improves primarily the calculation of intensity direction. Though requiring a greater computation time than the PAGE method, the CPAGE method is shown to improve intensity calculations, both in magnitude and direction.

Range estimation within a shallow-water waveguide via active and reactive acoustic intensity

The proposed ranging algorithm examines low-order modal variations in vertical reactive (Qz) and horizontal active (Ir) acoustic intensity and provides a gated estimate of the range from a receiver to a source. The receiver (measuring horizontal and vertical acoustic intensity) estimates range bins, for example, a source, which is confined within a range of 1 km to 2 km. Accurate initial range estimates provide key information for follow-on processors, such as tracking and depth classification algorithms. The technique relies on predictive broadband frequency characteristics between the superposition of low order trapped modes; modes which are restricted to the first few orders, for example, modes one through five. The binned range estimates can deliver, for example, initial values for higher level iterative and interpolative Detection, Classification and Localization (DCL) algorithms. Similarly, given an accurate opening estimate of a target’s range, neural networks techniques are enhanced. Existing at-sea shallow-water (<300 m), acoustic intensity data are compared to theoretical predictions to assess the feasibility and performance of the proposed algorithm.

The effects of contaminating noise on the calculation of active acoustic intensity for pressure gradient methods.

Bias errors for two-dimensional active acoustic intensity using multi-microphone probes have been previously calculated for both the traditional cross-spectral and the Phase and Amplitude Gradient Estimator (PAGE) methods [Whiting, Lawrence, Gee, Neilsen, and Sommerfeldt, J. Acoust. Soc. Am. 142, 2208-2218 (2017)]. Here, these calculations are expanded to include errors due to contaminating noise, as well as probe orientation. The noise can either be uncorrelated at each microphone location or self-correlated; the self-correlated noise is modeled as a plane-wave with a varying angle of incidence. The intensity errors in both magnitude and direction are dependent on the signal-to-noise ratio (SNR), frequency, source properties, incidence angles, probe configuration, and processing method. The PAGE method is generally found to give more accurate results, especially in direction; however, uncorrelated noise with a low SNR (below 10-15 dB) and low frequency (wavelengths more than 1/4 the microphone spacing) can yield larger errors in magnitude than the traditional method-though a correction for this is possible. Additionally, contaminating noise does not necessarily impact the possibility of using the PAGE method for broadband signals beyond a probe's spatial Nyquist frequency.

Using coherence to improve calculation of active acoustic intensity

The Phase and Amplitude Gradient Estimator (PAGE) method was developed as an alternative to the traditional method for calculating energy-based acoustic measures such as active acoustic intensity [E. B. Whiting et al. J. Acoust. Soc. Am. 142, 2208–2218 (2017)]. While this method shows many marked improvements over the traditional method, such as a higher valid frequency bandwidth for broadband sources, contaminating noise can lead to inaccurate results. Both the traditional method and the PAGE method can perform poorly when microphone pairs exhibit low coherence, whether caused by noise or by imprecise measurements. By using a coherence weighting in the least-squares pressure gradient calculations, where the weighting can vary across frequency, better estimates of the pressure gradient can be obtained, which in turn yield more accurate results for intensity. Additionally, a coherence-based approach may be used to mitigate the negative impact of contaminating noise, most especially for uncorrelated contaminating noise. These improvements, though requiring a greater amount of computation time, can be integrated into the PAGE method processing procedures to create a more stable method for calculating acoustic intensity. [Work supported by NSF.]

Active acoustic intensity errors due to contaminating noise

Bias errors for both one and two-dimensional intensity probes have been previously calculated for both the traditional cross-spectral and the Phase and Amplitude Gradient Estimator (PAGE) methods [E. B. Whiting et al., J. Acoust. Soc. Am.142, 2208–2218 (2017); E. B. Whiting, M. S. Thesis (2016)]. Here, these calculations are expanded to include errors due to the source angle relative to the probe and due to contaminating noise. The noise can either be uncorrelated at each microphone or self-correlated, which is modeled as a plane wave with a varying angle of incidence. The errors in both intensity magnitude and direction are dependent on the signal-to-noise ratio (SNR), frequency, source properties, incidence angles, probe configuration, and processing method. The PAGE method is generally found to give more accurate results, though uncorrelated noise with a low SNR can yield large errors. For broadband signals, the PAGE method can be used beyond the spatial Nyquist frequency. [Work supported by NSF.]

Experimental validation of acoustic intensity bandwidth extension by phase unwrapping.

The phase and amplitude gradient estimator (PAGE) method for active acoustic intensity uses pairwise microphone transfer functions to obtain the phase gradient, which improves the calculation bandwidth over the traditional weighted quadspectral method. Additionally, for broadband sources, the PAGE theory indicates that the transfer function phase can be unwrapped to further extend the usable frequency range to beyond the spatial Nyquist frequency. Here, two experiments demonstrate intensity bandwidth extension by more than an order of magnitude using phase unwrapping. First, plane-wave tube results show accurate one-dimensional intensity calculations with the microphones separated by five wavelengths, 30 times the traditional limit. Second, two-dimensional measurements of a laboratory-scale jet with a four-microphone probe yield physically reasonable calculations at frequencies 15 times the traditional limit.

Complex acoustic intensity transmission through the air-water interface

Sound transmission through the air-water interface occurs when there is a non-zero normal component of active and reactive acoustic intensity. Transmission occurs for both active intensity (pressure and particle velocity in phase) and reactive intensity (pressure and particle velocity out of phase), and depending on the proximity of the source to the surface these can convert from one form to the other. Intensity transmission through the interface is governed Chapman’s Law—which differs slightly from Snell’s law but is consistent with the requirement of phase continuity along the surface. Properties of intensity transmission through the air-water interface are examined using three examples: (1) Marine pile driving, where sound generated by the in-air portion of the pile is injected into the water-column, and sound generated underwater leaks into the air. (2) Transmission of far-field active and reactive intensity generated by aircraft, and how Chapman’s law or Snell’s law describe Doppler shifts. (3) The ability of a fish-catching bat to locate fish underwater with sonar, and how transmitted active intensity resonates with the swim-bladder of the fish generating a near-field that radiates into the air as active intensity. [Research supported by ONR.]

Numerical analysis of the vibration and noise induced by the unsteady flow in a centrifugal compressor

Flow-induced vibration and noise widely exist in turbomachines and the connecting piping systems. The combined numerical method of computational fluid dynamics, computational structural dynamics, and computational vibroacoustics is employed to investigate the vibration and noise induced by the unsteady flow in the centrifugal compressor and pipes. Computational results indicate that the strongest pressure fluctuation is located on the clearance between the impeller and the stationary units due to the periodic rotor–stator interaction. However, the intense vibration occurs on the inlet and outlet pipes of the centrifugal compressor. Moreover, the computational result of the normal active acoustic intensity shows that the primary vibro-noise source is located on the outlet pipe. The computational result also clarifies that the thickness of the outlet pipe is one of the key parameters affecting the acoustic power output of the centrifugal compressor.

Using vector sensors to measure the complex acoustic intensity field

The acoustic intensity vector field, defined as the product of pressure particle velocity fields, describes the flow of energy from an acoustic source. Following a brief introduction of acoustic vector sensors, which includes some direct measurements of acoustic intensity, the intensity field is shown to be composed of active and reactive components. Active intensity streamlines depict the time-averaged flow of acoustic energy and reveal characteristics of acoustic propagation including environmental influences. These streamlines do not characterize reactive intensity, which corresponds to the portion of acoustic intensity that time-averages to zero. Reactive intensity is significant in the near-field of a source and in environments where multipath interfere occurs. To examine the interplay between active and reactive acoustic intensity, the acoustic field generated by an airborne source positioned well above a water surface is presented. Acoustic measurements of a passing airplanes, made simultaneously above and below this sea-surface, are used to demonstrate properties of active and reactive intensity, including how reactive intensity can serve as an indicator of source altitude and range.

Computation of instantaneous and time-averaged active acoustic intensity field around rotating source

A vector aeroacoustics method is developed to analyze the acoustic energy flow path from the rotating source. In this method, the instantaneous and time-averaged active acoustic intensity vectors are evaluated from the time-domain and frequency-domain acoustic pressure and acoustic velocity formulations, respectively. With the above method, the acoustic intensity vectors and the acoustic energy streamlines are visualized to investigate the propagation feature of the noise radiated from the monopole and dipole point sources and the rotor in subsonic rotation. The result reveals that a portion of the acoustic energy spirals many circles before moving towards the far field, and another portion of the acoustic energy firstly flows inward along the radial direction and then propagates along the axial direction. Further, an acoustic black hole exists in the plane of source rotation, from which the acoustic energy cannot escape once the acoustic energy flows into it. Moreover, by visualizing the acoustic intensity field around the rotating sources, the acoustic-absorption performance of the acoustic liner built in the casing and centerbody is discussed.

Active average intensity based on single vector sensor

Code divided multiple access underwater communication based on single vector sensor is studied in this paper. The most common methods to estimate azimuth with self-directivity of single vector sensor are average sound intensity method and complex sound intensity method, however, for the same frequency band multi-users, theoretical limitation for these methods is only two users.Spread spectrum communication is featured with strong anti-multipath, anti-interference, secret-keeping and communication web composing ability. Active average intensity method, which measures azimuths of multi-users simultaneously with the excellent correlative characteristics of pseudo-random code in spread spectrum communication, is proposed in this paper. Simulation and experiment for same frequency band spread spectrum multi-user communication testify the feasibility and utility of active average sound intensity method. With the estimated azimuth, vector combination can be generated to adjust the directivity of vector sensor, achieve multi-user beam communication, inhibit multi-path interference, and enhance processing gain and lower error rate. Key words: underwater acoustic communication; CDMA; single vector sensor; active acoustic intensity average

Comparison of methods for processing acoustic intensity from orthogonal multimicrophone probes

One design for three-dimensional multimicrophone probes is the four-microphone orthogonal design consisting of one microphone at an origin position with the other three microphones equally spaced along the three coordinate axes. Several distinct processing methods have been suggested for the estimation of active acoustic intensity with the orthogonal probe; however, the relative merits of each method have not been thoroughly studied. This comparative study is an investigation of the errors associated with each method. Considered are orthogonal probes consisting of matched point sensor microphones both freely suspended and embedded on the surface of a rigid sphere. Results are given for propagating plane-wave fields for all angles of incidence. It is shown that the lowest error for intensity magnitude results from having the microphones in a sphere and using just one microphone for the pressure estimate. For intensity direction, the lowest error results from having the microphones in a sphere and using Taylor approximations to estimate the particle velocity and pressure.

Sound radiation from Caribbean steelpans using nearfield acoustical holography

The sound-field of a Caribbean low tenor steelpan is examined using nearfield acoustical holography (NAH). The particular instrument, a Bertrand Kelman low tenor, has been previously examined in a single plane using a four microphone sound intensity technique. This paper will examine the sound radiation in a cuboid space enclosing the instrument at three frequencies. Presented will be the results of sound pressure level (SPL), active acoustic intensity (AI), and reactive acoustic intensity (RI) obtained through application of the NAH technique. The limitations of NAH when applied to an irregularly shaped object such as a tenor steelpan will also be presented.

Code divided multiple access underwater acoustic communication based on active acoustic intensity average

Code divided multiple access (CDMA) underwater communication based on single vector sensor is studied in this paper. The most common methods to estimate azimuth with self-directivity of single vector sensor are average sound intensity method and complex sound intensity method, however, for the same frequency band multi-users, these methods are limited to the measurement of azimuths only for two users at the same time theoretically. An active average intensity method, which measures the azimuths of multi-users simultaneously with the excellent correlative characteristics of pseudo-random code in spread spectrum communication, is proposed in this paper. With the estimated azimuth, vector combination can be generated to adjust the directivity of vector sensor, achieve multi-user beam communication, inhibit multi-path interference, and enhance processing gain and lower error rate. Simulation and experiment for same frequency band spread spectrum multi-user communication testify the feasibility and utility of active average sound intensity method.

A characterization of scatterred acoustic intesity fields in the resonance region.

In this study, we investigate the properties of the scattered acoustic vector fields generated by simple rigid and fluid-filled spheroids. Analytical solutions are derived from acoustic target strength scattering models in the near field region. Of particular interest is the understanding of the characteristics of energy flow of the scattered acoustic vector field in the near- to far-field transition region. We utilize the separable active and reactive acoustic intensity fields to investigate the structural features of the scattered field components. Numerical and in-air measured results are presented for the near and transition region scattered acoustic vector field of simple spheroids. A qualitative method is developed for characterizing submerged scatterer properties based on boundary condition and features of the scattered vector field.

Studies of acoustic particle velocity measurements in the ocean.

This presentation reviews previously published results from applying various signal and array processing methods to the analysis of simultaneous measurements of acoustic particle velocity and acoustic pressure in the ocean. Particular focus is placed on interpreting the properties of the elements of the pressure/particle velocity data cross spectral matrix. These properties are used to evaluate measurements of the deep ocean infrasonic noise field during a period of rapidly changing wind speed. Numerical modeling of the evolution in directionality of the active and reactive acoustic intensity vectors during this time period demonstrates the influence of the continental slope on the flow of underwater acoustic energy. [Work supported by the Office of Naval Research.]

Characterization of the Near Scattered Acoustic Vector Field

In this study, we investigate the properties of the scattered acoustic vector fields generated by simple geometric objects, including the infinite rigid plate, disk, and sphere. Analytical solutions are derived from acoustic target strength scattering models in the near field region. Of particular interest is the understanding of the characteristics of energy flow of the scattered acoustic vector field in the near to far-field transition region. We utilize the time and space separable instantaneous active and reactive acoustic intensity to investigate the relative phase properties of the scattered field. Numerical results are presented for the near region scattered acoustic vector field of simple objects in both two and three dimensions.

A systematic method to obtain 3D finite-difference formulations for acoustic intensity and other energy quantities

A systematic method is described to obtain formulations based on the finite-difference approximation for computation of the energy quantities of three-dimensional (3D)-sound fields from measurements. It uses up to the first-order terms of the Taylor series expansion of the sound pressure about the microphone positions of a particular probe configuration. The use of a symbolic computation allows approximate expressions to be obtained for all energy quantities as a function of the cross-spectral densities of the microphone signals. The application of this approach is illustrated by the three components of the active acoustic intensity vector and the energy densities for different types of 3D-probes consisting of 4–6 microphones. The inherent effect associated with the finite-sum and finite-difference approximations is evaluated for each of these probes by considering 1D sound fields in order to make a comparison of their performance.

Changes in ocean infrasonic energy density and vector intensity with changes in wind and ocean surface wave conditions as measured by a freely drifting vector sensor

During an experiment in 1.3-km-deep water in the Southern California Bight, the wind speed decreased from about 7 to 1 m/s over a 10-h period, and then increased to 9 m/s over the next 8-h period. Measurements made by a neutrally buoyant, freely drifting vector sensor at 920-m depth show that the corresponding changes in the potential and kinetic energy density levels of the infrasonic sound field have a frequency-dependent delay. The levels in a 0.1-Hz-wide bin at 3 Hz respond to wind-speed changes with approximately a 2-h delay, and the delay in response increases with decreasing frequency at a rate of about 1 h per hertz down to 1 Hz. The corresponding evolution in directionality of the active and reactive acoustic intensity vectors in these same frequency bins and their relationship to the surrounding bathymetry will be presented. The directional spectra of the ocean surface wave field during the experiment were reconstructed using back-refraction modeling with wave data from shore stations and offshore platforms archived by the Coastal Data Information Program to obtain estimates of the temporal variability of the nonlinear wave-wave interactions generating the ocean infrasound. [Work supported by ONR.]